X-ray imaging apparatus and method of X-ray imaging

ABSTRACT

Provided is an X-ray imaging apparatus and a method of X-ray imaging, with which the apparatus can be reduced in size and a with which differential phase image or a phase image with consideration of an X-ray absorption effect of an object can be obtained. 
     A displacement of X-rays that have been split by a splitting element and have passed through an object is measured. The displacement can be measured by using a first attenuation element having a transmission amount that continuously changes in accordance with the incident position of X-rays. At this time, an X-ray transmittance of an object that is calculated by using a second attenuation element having a transmission amount that does not change in accordance with the incident position of X-rays is used.

TECHNICAL FIELD

The present invention relates to an X-ray imaging apparatus and a method of X-ray imaging.

BACKGROUND ART

Nondestructive testing using electromagnetic radiation is used for a variety of industrial applications and medical applications. X-rays are electromagnetic waves having a wavelength in the range of, for example, about 1 pm to 10 nm (10⁻¹² to 10⁻⁸ m). X-rays having a short wavelength (energy of about 2 keV or larger) are called hard X-rays, and X-rays having a long wavelength (energy in the range of about 0.1 keV to about 2 keV) are called soft X-rays.

For example, an absorption contrast method obtains an absorption image of an object by using the difference in transmittance of X-rays passing through the object. Because X-rays easily penetrate through an object, the absorption image is used for internal crack detection of steel materials and for security purposes such as baggage inspection.

On the other hand, an X-ray phase imaging method detects phase shift of X-rays caused by an object. The X-ray phase imaging method is effective for an object made of a material having a small density difference, because absorption contrast of X-rays is not clear for such a material. For example, the X-ray phase imaging method can be used for imaging a phase separation structure made of polymer blends or for medical applications.

Patent Literature 1 describes a very convenient and effective method of X-ray phase imaging that uses a refraction effect due to phase shift of X-rays caused by an object. To be specific, the method uses a microfocus X-ray source and utilizes an effect that the edge of an object is detected in an enhanced manner owing to a refraction effect of X-rays caused by the object when the distance between the object and a detector is large. Because the method uses the refraction effect, it is not necessary to use highly coherent X-rays such as synchrotron radiation, which distinguishes the method from many other X-ray phase imaging methods.

Patent Literature 2 discloses an X-ray imaging apparatus having a mask that blocks X-rays on an edge portion of the pixel of the detector. By setting the mask so that a part of the mask is irradiated with X-rays when an object is not present, a displacement of X-rays due to a refraction effect caused by the object can be detected as a change in intensity.

CITATION LIST

Patent Literature

PTL 1 Japanese Patent Laid-Open No. 2002-102215

PTL 2 International Publication No. WO2008/029107

SUMMARY OF INVENTION Technical Problem

However, in order to obtain an edge-enhanced image of an object by using the method described in Patent Literature 1, it is necessary that the distance between the object and the detector be sufficiently large with consideration of the size of a pixel of the detector, because the refraction angle due to the refraction effect of X-rays caused by the object is very small. Therefore, the method described in Patent Literature 1 requires a large apparatus.

In contrast, the distance between an object and a detector may be small with the method described in Patent Literature 2, so that a small apparatus can be used. However, because a mask that blocks X-rays is provided, a displacement of X-rays incident on the blocking mask cannot be detected. That is, high-precision analysis is difficult because an insensitive region exists.

Moreover, the methods described in the Patent Literature 1 and Patent Literature 2 have a problem in that an absorption effect of X-rays and a phase effect of X-rays cannot be obtained separately if an object absorbs a large amount of X-rays.

The present invention provides an X-ray imaging apparatus and a method of X-ray imaging for obtaining a differential phase image or a phase image with consideration of an absorption effect of X-rays caused by the object, in which the size of the apparatus can be made smaller than that of the method described in Patent Literature 1, and with which an analysis having a precision higher than that of the method described in Patent Literature 2 can be performed.

Solution to Problem

An X-ray imaging apparatus according to an aspect of the present invention includes a splitting element that spatially splits X-rays generated by an X-ray generator; a first attenuation element on which X-rays split by the splitting element are incident; a second attenuation element on which X-rays split by the splitting element are incident, the second attenuation element being disposed adjacent to the first attenuation element; and a detecting unit configured to detect intensities of X-rays that have passed through the first attenuation element and the second attenuation element, wherein the first attenuation element is configured so that a transmission amount of X-rays continuously changes in accordance with a position at which the X-rays are incident, and wherein the second attenuation element is configured so that a transmission amount of X-rays does not change in accordance with a position at which the X-rays are incident.

A method of X-ray imaging used in an X-ray imaging apparatus, the method according to an aspect of the present invention, includes generating X-rays; spatially splitting the X-rays; making X-rays that have been spatially split be incident on a first attenuation element and a second attenuation element, the second attenuation element being disposed adjacent to the first attenuation element; detecting intensities of X-rays that have passed through the first attenuation element and the second attenuation element; and calculating a differential phase image or a phase image of an object using an X-ray transmittance of the object, the X-ray transmittance being calculated from the intensities of X-rays that have been detected, wherein the first attenuation element is configured so that a transmission amount of X-rays continuously changes in accordance with a position at which the X-rays are incident, and wherein the second attenuation element is configured so that a transmission amount of X-rays does not change in accordance with a position at which the X-rays are incident.

Advantageous Effects of Invention

The present invention provides an X-ray imaging apparatus and a method of X-ray imaging for obtaining a differential phase image or a phase image with consideration of an absorption effect of X-rays caused by the object, in which the size of the apparatus can be made smaller than that of the method of Patent Literature 1, and with which an analysis having a precision higher than that of the method of Patent Literature 2 can be performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an apparatus according to first and second embodiments.

FIG. 2 is a schematic view of an attenuation unit included in the apparatus according to the first embodiment.

FIG. 3 is a flowchart of a process performed by a calculating unit included in the apparatus according to the first embodiment.

FIG. 4 is a schematic view of an attenuation unit included in an apparatus according to the second embodiment.

FIG. 5 is a schematic view of a CT apparatus according to a third embodiment.

FIG. 6 is a flowchart of a process performed by a calculating unit included in an apparatus according to the third embodiment.

FIG. 7 is a schematic view of an apparatus described as an example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, X-ray imaging apparatuses according to embodiments of the present invention will be described. Each of the apparatuses uses a phase shift of X-rays caused by an object, and is capable of obtaining a more precise X-ray differential phase image of an object even if the object has a high absorptance of X-rays.

To be specific, the apparatus includes first attenuation elements, each having an absorptance gradient (transmittance gradient), so as to convert a displacement of the incident position of X-rays, which is due to the refraction effect of the X-rays caused by an object, into intensity information of the X-rays and thereby detect the displacement. Moreover, the apparatus includes second attenuation elements so as to detect a change in the intensity of transmitted X-rays due to an absorption effect of the X-rays caused by the object.

An attenuation element (the first attenuation element) having an absorptance gradient (transmittance gradient) refers to an element in which the amount of X-rays absorbed (transmitted) continuously changes in accordance with the incident position of the X-rays. The attenuation element can be made by changing the shape continuously or stepwise. Alternatively, the attenuation element can be made by changing an amount of X-rays absorbed (transmitted) per unit volume continuously or stepwise. In this description, the term “continuously” may include the meaning of “stepwise”.

An attenuation element (the second attenuation element) for detecting a change in the intensity of transmitted X-rays caused by the absorption effect of the object refers to an element in which the amount of X-rays transmitted does not change when the incident position of the X-rays is changed. The expression “an element in which the amount of X-rays transmitted does not change” means that “an element in which the amount of X-rays transmitted does not substantially change”. An example of such an element is an element in which the variation in the detected intensity of X-rays remains approximately within the variation in the detected intensity with respect to the statistical variation in the number of photons of the X-rays, when the incident position of the X-rays is changed. A major factor responsible for the detection variation in an X-ray detector is the statistical variation in the incident X-rays. In general, the statistical variation in the number of photons of X-rays has a Poisson distribution. The above-described element is desirable because the quantitative properties of a transmission image and a differential phase image deteriorate if the variation in the detected intensity of X-rays with respect to the incident position of the X-rays is large.

If, for example, the first attenuation elements are discretely disposed on a substrate, regions of the substrate on which the first attenuation elements are not disposed serve as the second attenuation elements.

The second attenuation elements may be openings formed in the substrate, because the second attenuation elements are elements in which the transmission amount of X-rays does not substantially change when the incident position of the X-rays changes. In this case, X-rays can be effectively used, because the X-rays are not attenuated by the second attenuation elements.

With the above-described structure, an X-ray transmittance distribution (transmittance image) of an object can be obtained by detecting the intensity of X-rays that have passed through the second attenuation elements. By using the information about the transmittance, a more precise differential phase image or a phase image can be obtained. Specific description will be given below.

First Embodiment

An X-ray imaging apparatus according to a first embodiment obtains an absorption image from a change in the absorption of X-rays, that is, from a change in the transmittance of X-rays. Moreover, the X-ray imaging apparatus obtains a differential phase image and a phase image from a phase shift of X-rays.

FIG. 1 is a schematic view of an X-ray imaging apparatus according to the first embodiment. An X-ray source 101 generates X-rays. On the optical path of the X-rays, a splitting element 103, an object 104, an attenuation unit 105, and a detector 106 are disposed.

Movement units 109, 110, and 111, such as stepping motors, may be additionally provided so as to move the splitting element 103, the object 104, and the attenuation unit 105. The object 104 can be appropriately moved so that an image of a specific position of the object 104 can be obtained. X-rays that have been generated by the X-ray source 101 are spatially split by the splitting element 103. That is, the splitting element 103 serves as a sample mask having a plurality of apertures described in Patent Literature 2, and the X-rays that have passed through the splitting element 103 become a bundle of X-rays. The splitting element 103 may have a slit array with a pattern of lines and spaces, or may have holes that are arranged two-dimensionally.

As long as X-rays can pass through the slits in the splitting element 103, it is not necessary that the slits extend through the substrate of the optical element. The material of the splitting element 103 can be selected from substances having a high X-ray absorptance, such as Pt, Au, Pb, Ta, and W.

The intervals between lines and spaces of the X-rays, which have been split by the splitting element 103, at the position of the detector 106 are equal to or larger than the pixel size of the detector 106. That is, the size of pixels that constitute an X-ray intensity detecting unit is equal to or smaller than the spatial period of the X-rays at the position of the detector 106.

The object 104 absorbs the sheet-shaped X-rays that have been spatially split by the splitting element 103, changes the phases of the X-rays, and refracts the X-rays. The X-rays that have been refracted are incident on the attenuation unit 105. The detector 106 detects the intensities of the X-rays that have passed through the attenuation unit 105. A calculating unit 107 numerically processes the information about the X-rays obtained by the detector 106, and outputs the result of the processing to a display unit 108.

Examples of the object 104 include a human body, an inorganic material, and an inorganic-organic composite material.

The detector 106 can be selected from, for example, an X-ray flat panel detector, an X-ray CCD camera, and a direct-conversion-type two-dimensional X-ray detector. The detector 106 may be disposed adjacent to the attenuation unit 105, or disposed at a distance from the attenuation unit 105. Alternatively, the attenuation unit 105 may be incorporated in the detector 106.

In order to use monochromatic X-rays, a monochromatizing unit 102 may be disposed between the X-ray source 101 and the splitting element 103. The monochromatizing unit 102 may be a combination of a monochromator and slits, or an X-ray multilayer mirror. In order to reduce blurring of an image caused by scattered X-rays from the attenuation unit 105, a grid used for radiography may be disposed between the attenuation unit 105 and the detector 106.

FIG. 2 is a partial schematic view of the attenuation unit 105. A reference X-ray 201 is an X-ray that has been split when the object 104 is not present. An X-ray 202 is an X-ray that has been refracted by the object 104.

An attenuation unit 203 includes attenuation elements 204 (first attenuation elements) and attenuation elements 205 (second attenuation elements) that are alternately arranged adjacent to one another.

As illustrated on the right-hand side of FIG. 2, each of the attenuation elements 204 has a linear density distribution with respect to the X direction (a direction perpendicular to incident X-rays). On the other hand, each of the attenuation elements 205 has a constant density distribution with respect to the X direction.

That is, each of the attenuation elements 204 has a density variation that changes the degree of absorption of X-rays. The higher the density, the less transparent the attenuation element 204 is to X-rays. In other words, the attenuation element 204 has an absorptance gradient that changes the absorption amount (transmission amount) of X-rays in accordance with the displacement of the X-rays in the attenuation element 204.

The intensity of the reference X-ray 201 that has passed through the attenuation element 204 is expressed by equation (1).

$\begin{matrix} {I = {I_{0}{\mathbb{e}}^{{- {(\frac{\mu}{\rho})}}\rho_{0}L}}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \end{matrix}$ I₀ is the intensity of X-rays that have been spatially split by the splitting element 103, μ/ρ is the effective mass absorption coefficient of the attenuation element 204, ρ₀ is the density of the attenuation element 204 at a portion of the attenuation element 204 through which the reference X-ray 201 has passed, and L is the thickness of the attenuation element 204.

The intensity the X-ray 202 that has been refracted by the object 104 and passed through the attenuation element 204 is expressed by equation (2).

$\begin{matrix} {I^{\prime} = {I_{0}A\;{\mathbb{e}}^{{- {(\frac{\mu}{\rho})}}\rho^{\prime}L}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack \end{matrix}$

A is the X-ray transmittance of the object 104, and ρ′ is the density of the attenuation element 204 at a portion of the attenuation element 204 through which the X-ray 202 has passed. From equations (1) and (2), the difference in the density of the attenuation element 204 with respect to the reference X-ray 201 and the X-ray 202 is expressed by equation (3).

$\begin{matrix} {{\rho_{0} - \rho^{\prime}} = {\frac{1}{\left( \frac{\mu}{\rho} \right)L}{\ln\left( \frac{I^{\prime}}{I\; A} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack \end{matrix}$

The X-ray transmittance A of the object 104 can be obtained from the ratio of the intensity of the reference X-ray to the intensity of the refracted X-ray, the reference X-ray and the refracted X-ray having passed through the attenuation element 205 adjacent to the attenuation element 204.

Because the density distribution of an X-ray absorber is known, the displacement d of X-rays on the attenuation unit 105 can be obtained from the density difference expressed by equation (3). Otherwise, a table of the correspondence relation between the transmitted X-ray intensity and the displacement d can be stored in the calculating unit 107, storage, or the like. The displacement d can be estimated from the measured intensity using the data table. This data table can be prepared, for respective attenuation elements 204, by moving the attenuation unit 105 or the splitting element 103 and detecting the transmitted X-ray intensity at each of the positions. Also, in preparing the data table, a single slit having the same slit width as the splitting element 103 may be employed, and the transmitted X-ray intensity is detected at the respective positions of the attenuation elements 204. That is, by using the relationship between the detected intensities of the reference X-ray 201 and the X-ray 202, a slight displacement due to X-ray transmittance and refraction caused by the absorption effect of the object 104 can be obtained. In this case, the spatial resolution with respect to the X direction is halved, because the differential phase image and the like are formed by using information about the intensities of X-rays that have passed through two regions of the attenuation element 204 and the attenuation element 205.

In addition to the above-described measurement, measurement can be performed in a similar manner while moving the attenuation unit 105 or the object 104 in the X direction by the length of the attenuation element 204 with respect to the X direction using the movement unit 111 or the movement unit 110. Thus, information about the X-ray transmittance A corresponding to the position of the object 104 for which the displacement of the X-rays has been measured previously can be obtained.

By using the attenuation unit 105, information about the absorption effect of X-rays and information about the refraction effect of the X-rays can be independently obtained. Moreover, by using the attenuation unit 105, the displacement of X-rays that is equal to or smaller than the pixel size of the detector 106 can be detected, whereby the distance between the object and the detector can be reduced, so that the apparatus can be reduced in size.

FIG. 3 is a flowchart of a process performed by the calculating unit 107. First, information about the intensity of each X-ray that has passed through the attenuation unit 105 is obtained (S100).

Next, the X-ray transmittance A of the object 104 is calculated from the intensity of each X-ray, and the displacement d with respect to the reference X-ray 201 is calculated (S101).

The refraction angle Δθ of each X-ray is expressed by equation (4) by using the displacement d and the distance Z between the object 104 and the attenuation unit 105.

$\begin{matrix} {{\Delta\;\theta} = {\tan^{- 1}\left( \frac{d}{Z} \right)}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack \end{matrix}$

Using equation (4), the refraction angle Δθ of each X-ray is calculated (S102). The refraction angle Δθ and the differential phase dφ/dx have a relationship expressed by equation (5).

$\begin{matrix} {\frac{\mathbb{d}\phi}{\mathbb{d}x} = {\frac{2\pi}{\lambda}\Delta\;\theta}} & \left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack \end{matrix}$

λ is the wavelength of the X-rays. For continuous X-rays, the wavelength refers to the effective wavelength. Using equation (5), the differential phase dφ/dx of each X-ray is calculated (S103).

The differential phase dφ/dx is integrated in the X direction so as to calculate the phase φ (S104).

The transmittance image, the differential phase dφ/dx, and the phase φ, which have been calculated as described above, can be displayed on the display unit 108 (S105).

With this structure, a slight displacement within a small pixel of the detector 106 can be detected. Therefore, it is not necessary that the distance between the object 104 and the detector 106 be large, and the apparatus can be reduced in size. Moreover, an insensitive region does not exist, because the attenuation unit 105 is of a pass-through-type that does not have a region that blocks X-rays.

By increasing the distance between the object 104 and the detector 106, a smaller displacement of X-rays due to refraction can be measured.

With the above-described structure, because the phase shift is detected by using the refraction effect of X-rays, a differential phase image or a phase image with consideration of an absorption effect can be measured without necessarily using highly coherent X-rays.

In FIG. 2, the first attenuation element having an absorptance gradient has a density that changes continuously. However, the first attenuation element may have a density that changes stepwise.

In FIG. 2, the second attenuation element, in which the transmission amount of X-rays does not change in accordance with the displacement of X-rays, is an attenuation element that has a constant density. However, the second attenuation element may be an element that does not absorb X-rays, because it is sufficient that the transmission amount of X-rays does not change in the second attenuation element in accordance with the incident position of X-rays.

Second Embodiment

An apparatus according to a second embodiment includes an attenuation unit illustrated in FIG. 4 instead of the attenuation unit included in the first embodiment. The structure of the apparatus is the same as the first embodiment. The splitting element 103 spatially splits X-rays into sheet-shaped X-rays. The object 104 is irradiated with the X-rays, and the X-rays that have passed through the object 104 are incident on the attenuation unit 105. FIG. 4 is a partial schematic view of the attenuation unit 105.

A reference X-ray 501 is an X-ray that has been split when the object 104 is not present. The reference X-ray 501 can be incident on the middle position of one of attenuation elements 504 with respect to the X direction. An X-ray 502 is an X-ray refracted by the object 104. An attenuation unit 503 includes the attenuation elements 504 (first attenuation elements) and attenuation elements 505 (second attenuation elements) that are arranged at regular intervals. The attenuation elements 504 are triangular-prism-shaped structures.

FIG. 4 illustrates examples of the attenuation elements 505 that do not include X-rays absorbers.

Because the attenuation elements 504 are triangular prism shaped, the optical path lengths of X-rays that pass through the attenuation elements 504 change in the X direction. The intensity of the reference X-ray 501 that has passed through the attenuation element 504 is expressed by equation (6). I=I₀ e ^(−μl) ⁰   [Math. 6]

I₀ is the intensity of X-rays that have been spatially split by the splitting element 103, μ is the effective linear absorption coefficient of the attenuation element 504, and l₀ is the optical path length of the reference X-ray 501 in the attenuation element 504. The intensity the X-ray 502 that has been refracted by the object 104 and passed through the attenuation element 504 is expressed by equation (7). I′=I ₀ Ae ^(−μl)   [Math. 7]

A is the X-ray transmittance of the object 104, and l is the optical path length of the X-ray 502 in the attenuation element 504. Using equations (6) and (7) and the vertex angle α of the attenuation element 504, the displacement d on the attenuation unit 105 can be expressed by equation (8).

$\begin{matrix} {d = {\frac{1}{\mu}{\ln\left( \frac{I^{\prime}}{I\; A} \right)}\tan\;\alpha}} & \left\lbrack {{Math}.\mspace{14mu} 8} \right\rbrack \end{matrix}$

The X-ray transmittance A of the object 104 can be obtained from the ratio of the intensity of the reference X-ray to the intensity of the refracted X-ray, the reference X-ray and the refracted X-ray having passed through the attenuation element 505. The attenuation element 505, which is disposed adjacent to the attenuation element 504, has a constant X-ray transmittance. That is, by using the relationship between the detected intensities of the reference X-ray 501 and the X-ray 502, a small displacement due to the absorption effect of the object 104 and refraction caused by the object 104 can be obtained. Without Equation 8, the displacement d can be estimated from the table as described in the first embodiment.

In this case, the spatial resolution with respect to the X direction is halved, because the information about two regions of the attenuation element 505, which does not have a density variation and is disposed adjacent to the attenuation element 504, is used.

In addition to the above-described measurement, measurement can be performed in a similar manner while moving the attenuation unit 105 or the object 104 in the X direction by the length of the attenuation element 504 with respect to the X direction using the movement unit 111 or the movement unit 110. Thus, information about the X-ray transmittance A corresponding to the position of the object 104 for which the displacements of X-rays have been measured previously can be obtained.

Because the attenuation element 504 is triangular prism shaped, the displacement d can be determined from the ratio of the intensity of the reference X-ray 501 to the intensity of the X-ray 502 at any position on the attenuation element 504.

The X-rays that have passed through the attenuation unit 105 are detected by the X-ray detector 106. Using the detected data, the calculating unit 107, which is similar to that of the first embodiment, calculates the transmittance A, the differential phase dφ/dx, and the phase φ. The results of the calculation can be displayed on the display unit 108.

With this structure, a slight displacement within a small pixel of the detector 106 can be detected. Therefore, it is not necessary that the distance between the object 104 and the detector 106 be large, and the apparatus can be reduced in size. Moreover, an insensitive region does not exist, because the attenuation unit 105 is of a pass-through-type that does not have a region that blocks X-rays.

By increasing the distance between the object 104 and the detector 106, a smaller displacement of X-rays due to refraction can be detected.

With the above-described structure, because the phase shift is detected by using the refraction effect of X-rays, a differential phase image or a phase image with consideration of an absorption effect can be measured without necessarily using highly coherent X-rays.

In FIG. 4, the first attenuation element having an absorptance gradient has a shape that changes continuously. However, the first attenuation element may have a shape that changes stepwise.

In FIG. 4, the second attenuation element that does not have an absorptance gradient is an element that does not have an absorber. However, the second attenuation element may have an absorber having a shape that does not change with respect to a direction perpendicular to the incident X-rays. The absorber may be, for example, rectangular parallelepiped shaped.

Third Embodiment

An apparatus according to a third embodiment obtains a three-dimensional absorption distribution and a phase distribution by using the principle of computed tomography (CT).

FIG. 5 illustrates an apparatus according to the third embodiment. An X-ray source 401, a splitting element 403, an attenuation unit 405, and an X-ray detector 406 can be rotated around an object 404 by a movement unit in a synchronized manner.

The splitting element 403 spatially splits X-rays, the object 404 is irradiated with the X-rays, and the X-rays that have passed through the object 404 are incident on the attenuation unit 405.

Owing to the attenuation unit 405, the amount of the X-rays that have been split and absorbed by the object 404 and small displacements of the X-rays due to refraction can be obtained. The X-rays that have passed through the attenuation unit 405 are detected by the X-ray detector 406. A projection data of the object 404 can be obtained by performing imaging while the X-ray source 401, the splitting element 403, the attenuation unit 405, and the X-ray detector 406 are being rotated around the object 404 in a synchronized manner. The object 404 may be rotated so as to obtain a projection data, instead of rotating the splitting element 403, the attenuation unit 405, and the X-ray detector 406.

FIG. 6 is a flowchart of a process performed by a calculating unit 407. First, information about the intensity of each X-ray that has passed through the attenuation unit 405 is obtained (S200).

Next, the X-ray transmittance A and the displacement d with respect to the reference X-ray 501 are calculated from the information about the intensity of each X-ray (S201).

Using the displacement d and the distance Z between the object 404 and the attenuation unit 405, the refraction angle Δθ of each X-ray is calculated (S202).

The differential phase dφ/dx of each X-ray is calculated from the refraction angle Δθ (S203).

The differential phase dφ/dx is integrated in the X direction so as to calculate the phase φ (S204).

The series of operations (S201 to S204) are performed for all projection data. By applying an image reconstruction method (for example, a filter backprojection method) of computed tomography to the transmittance images and the phase images of all the projection data, tomographic images of the object 404 are obtained (S206).

The tomographic images can be displayed on a display unit 408 (S205).

With this structure, the CT apparatus can be reduced in size. Moreover, because the apparatus uses the refraction effect of X-rays, the CT apparatus can nondestructively obtain a three-dimensional absorption image and a phase image of an object without necessarily using highly coherent X-rays.

Other Embodiments

Each of the above-described embodiments includes attenuation elements each having an absorptance gradient (transmittance gradient) in one direction. However, the attenuation element may have an absorptance gradient (transmittance gradient) in more than one directions. For example, by providing the attenuation element with absorption gradients in the X and Y directions, two-dimensional phase gradient can be measured. Examples of a shape having gradients in the X and Y directions include a pyramidal shape and a conical shape.

Alternatively, a two-dimensional phase gradient can be detected using an attenuation unit including attenuation elements each having a gradient in the X direction and attenuation elements each having a gradient in the Y direction. As a further alternative, attenuation elements each having gradients in the X direction and in the Y direction may be stacked on top of one another.

As described above, the X-ray imaging apparatus according to an embodiment the present invention includes a first element configured to obtain first intensity data and a second element configured to obtain second intensity data, the second element being disposed adjacent to the first element. The X-ray imaging apparatus further includes a calculation unit that calculates the amount of phase shift of X-rays from the first intensity data on the basis of the X-ray transmittance of an object obtained from the second intensity data. The X-ray imaging method according to an embodiment of the present invention includes obtaining first intensity data using a first element and obtaining second intensity data using a second element. The method further includes obtaining the amount of phase shift of X-rays from the first intensity data on the basis of an X-ray transmittance of an object obtained from the second intensity data.

EXAMPLE

FIG. 7 is a schematic view of an apparatus described as an example. As an X-ray generator, an X-ray source 701 that is a Mo target rotating-cathode-type X-ray generating apparatus was used. A monochromator 702 made of high orientation pyrolytic graphite (HOPG) was used as a monochromatizing unit of X-rays so as to extract characteristic X-rays of Mo.

A splitting element 703 was disposed at a position that is 100 cm away from the X-ray source. The X-rays that had been monochromatized by the monochromator 702 were spatially split by the splitting element 703. As the splitting element 703, a tungsten (W) plate that has a thickness 100 μm and slits arranged therein, each of the slits having a width of 40 μm, was used. The intervals between the slits were 150 μm on an attenuation unit 705. Besides W, materials such as Au, Pb, Ta, and Pt can be used.

An object 704 was irradiated with X-rays split by the splitting element 703. The attenuation unit 705 was disposed at a position that is 50 cm away from the object 704. The X-rays that had passed through the object 704 were incident on the attenuation unit 705. Movement units 709, 710, and 711 using stepper motors were respectively provided to the splitting element 703, the object 704, and the attenuation unit 705.

The attenuation unit 705 had a structure in which triangular prisms and quadrangular prisms that are made of Ni and alternately arranged on a carbon substrate having a thickness of 1 mm. The length of a side of the base of each triangular prism was 150 μm, and the height of the triangular prism was 75 μm. The length of a side of the base of each quadrangular prism was 150 μm, and the height of the quadrangular prism was 75 μm. An X-ray detector 706, which served as a detecting unit, was disposed right behind the attenuation unit 705. The X-ray detector 706 detected the intensity of X-rays that had passed through the attenuation unit 705. Subsequently, the attenuation unit 705 was moved by 150 μm with the movement unit 711 in the direction in which the triangular prisms are arranged, and a similar measurement was performed. As the X-ray detector 706, a flat panel detector having a pixel size of 50 μm×50 μm was used. The sum of the intensities of X-rays for three pixels arranged in the direction in which the triangular prisms were arranged was used as the intensity of the X-rays for one attenuation element.

Using a change in the intensity of each X-ray from the intensity obtained when similar imaging was performed without the object 704, a calculating unit 707 calculated the X-ray transmittance A of the object 704, so that an absorption image was obtained. The calculating unit 707 also calculated the displacement d using equation (8) and the refraction angle Δθ using equation (4).

The amount of differential phase was calculated from the refraction angle Δθ using equation (5), and a phase distribution image was obtained by spatially integrating the amount of differential phase.

The X-ray transmittance image, the X-ray differential phase image, and the X-ray phase image, which had been calculated by the calculating unit 707, were displayed on a PC monitor that served as a display unit 708.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2009-006848 filed Jan. 15, 2009 and No. 2009-264412 filed Nov. 19, 2009, which are hereby incorporated by reference herein in their entirety.

Reference Signs List

-   101 X-ray source -   102 monochromatizing unit -   103 splitting element -   104 object -   105 attenuation unit -   106 detector -   107 calculating unit -   108 display unit -   109 movement unit -   110 movement unit -   111 movement unit -   201 reference X-ray -   202 X-ray -   203 attenuation unit -   204 attenuation element -   205 pass through region 

The invention claimed is:
 1. An X-ray imaging apparatus comprising: a splitting element that spatially splits X-rays generated by an X-ray generator; a first attenuation element on which X-rays split by the splitting element and passed through an object are incident; a second attenuation element on which X-rays split by the splitting element and passed through the object are incident, the second attenuation element being disposed adjacent to the first attenuation element; and a detecting unit configured to detect intensities of X-rays that have passed through the first attenuation element and the second attenuation element, wherein the first attenuation element is configured so that a transmission amount of X-rays continuously changes in accordance with a position at which the X-rays are incident, and wherein the second attenuation element is configured so that a transmission amount of X-rays does not change in accordance with a position at which the X-rays are incident.
 2. The X-ray imaging apparatus according to claim 1, further comprising: a calculating unit configured to calculate a differential phase image or a phase image of an object using an X-ray transmittance that is calculated from the intensities of X-rays detected by the detecting unit.
 3. The X-ray imaging apparatus according to claim 2, wherein the calculating unit calculates a transmittance image of the object from the intensities of X-rays detected by the detecting unit.
 4. The X-ray imaging apparatus according to claim 1, wherein a thickness of the second attenuation element is constant in a direction perpendicular to a direction of X-rays incident on the second attenuation element.
 5. The X-ray imaging apparatus according to claim 1, wherein a density of the second attenuation element is constant in a direction perpendicular to a direction of X-rays incident on the second attenuation element.
 6. The X-ray imaging apparatus according to claim 1, wherein the second attenuation element is a substrate on which the first attenuation element is disposed.
 7. The X-ray imaging apparatus according to claim 1, wherein the second attenuation element is an opening formed in a substrate on which the first attenuation element is disposed.
 8. The X-ray imaging apparatus according to claim 1, wherein a thickness of the first attenuation element continuously changes in a direction perpendicular to a direction of X-rays incident on the first attenuation element.
 9. The X-ray imaging apparatus according to claim 8, wherein the first attenuation element is a structure having a shape of a triangular prism.
 10. The X-ray imaging apparatus according to claim 1, wherein a density of the first attenuation element continuously changes in a direction perpendicular to a direction of X-rays incident on the first attenuation element.
 11. A method of X-ray imaging used in an X-ray imaging apparatus, the method comprising: generating X-rays; spatially splitting the X-rays; making X-rays that have been spatially split be incident on a first attenuation element and a second attenuation element, the second attenuation element being disposed adjacent to the first attenuation element; detecting intensities of X-rays that have passed through the first attenuation element and the second attenuation element; and calculating a differential phase image or a phase image of an object using an X-ray transmittance of the object, the X-ray transmittance being calculated from the intensities of X-rays that have been detected, wherein the first attenuation element is configured so that a transmission amount of X-rays continuously changes in accordance with a position at which the X-rays are incident, and wherein the second attenuation element is configured so that a transmission amount of X-rays does not change in accordance with a position at which the X-rays are incident.
 12. An X-ray imaging apparatus comprising: a splitting element that spatially splits the X-rays generated by an X-ray generator into first X-rays and second X-rays in a first direction perpendicular to a beam axis of X-rays; a first attenuation element on which the X-rays split by the splitting element and passed through an object are incident; a second attenuation element on which the X-rays split by the splitting element and passed through the object are incident, the second attenuation element being disposed in the first direction adjacent to the first attenuation element; and a detecting unit configured to detect first intensities of the first X-rays that have passed through the first attenuation element and second intensities of the second X-rays that have passed through the second attenuation element, wherein the first attenuation element is configured so that its X-ray transmittance changes in the first direction in accordance with a position on the first attenuation element, and wherein the second attenuation element is configured so that its X-ray transmittance does not change in the first direction in accordance with a position on the second attenuation element.
 13. The X-ray imaging apparatus according to claim 12, wherein the first attenuation element is configured so that its X-ray transmittance continuously changes in the first direction in accordance with a position on the first attenuation element.
 14. The X-ray imaging apparatus according to claim 12, further comprising: a calculating unit configured to calculate a differential phase image or a phase image of an object from the first and second intensities.
 15. The X-ray imaging apparatus according to claim 14, wherein the calculating unit is configured to calculate a transmittance image of the object from the second intensity of the second X-rays detected by the detecting unit.
 16. The X-ray imaging apparatus according to claim 12, wherein the second attenuation element is a substrate on which the first attenuation element is disposed.
 17. The X-ray imaging apparatus according to claim 12, wherein the second attenuation element is an opening formed in a substrate on which the first attenuation element is disposed.
 18. The X-ray imaging apparatus according to claim 12, wherein the first X-rays passed through the object are incident on the plurality of first elements, and the second X-rays passed through the object are incident on the plurality of second elements. 